1. Field of the Invention
The present invention generally relates to devices for diverting particulate matter away from spaces around and between catalyst blocks used in selective catalytic reduction (SCR) devices to remove nitrogen oxides (NOx) from flue gases and, in particular, to particulate diverter devices for diverting powdered and/or solid particulate matter away from spaces around and between catalyst blocks used in such SCRs.
2. Description of the Related Art
The by-products of fossil fuel combustion include, among other things, flue gases and generally an ash component. The ash component of a fossil fuel combustion process is particularly notable in the case of coal combustion, and to a lesser extent in the combustion of oil. The combination of the ash and gaseous combustion products can, if discharged in sufficient quantity, cause significant air pollution. The ash component of such flue gases generally contain silicon dioxide, aluminum oxide, and ferric oxide ash particles, whereas the gaseous component of the flue gases contain, at a minimum, carbon dioxide, sulfur oxides (SOx), and nitrogen oxides (NOx).
A variety of methods and/or devices are used to remove, for example, particulates, sulfur oxides (SOx) and nitrogen oxides (NOx). The problems of particulate or ash removal and disposal from flue gases can be significant in the case of the combustion of coal, since about 80% to about 90% of the ash generated by coal combustion processes remains in the flue gases. Current energy policy in the United States is based on expanded use of coal in utility and industrial applications. Such expanded use of coal can not lead to a compromise in environmental requirements for clean air. Accordingly, advanced control technologies are needed to control the increase in pollutant emissions of coal combustion.
Electrostatic precipitators and other particulate collection devices such as fabric filter houses (baghouses) have been used to effectively remove such particulates from flue gases prior to atmospheric release. U.S. Pat. No. 4,309,386 discloses a hot catalytic baghouse which simultaneously removes particulate material and reduces the NOx emissions. This patent describes coating the catalyst onto the fabric of the baghouse filter media including the use of a fabric filter in which the catalyst is woven into the fabric. U.S. Pat. No. 4,793,981 describes the use of a hot catalytic baghouse which simultaneously collects sulfur oxides, nitrogen oxides and particulates.
U.S. Pat. No. 4,602,673 discloses an apparatus for pre-heating combustion air while simultaneously reducing NOx contained in the flue gas. By combining a catalytic reactor with an air heater, a compact device is possible according to that patent. However, the catalyst will have to be replaced frequently because of erosion of the catalyst due to ash and/or dust build-up. In addition, fly ash erosion will reduce the effective life of an SCR catalyst.
SCRs using catalyst blocks are often used in applications where NOx removal from flue gases is needed. In such cases, the SCR catalyst blocks are placed within the SCR reactor through which the combustion flue gases are conveyed. In order to remain effective, the catalyst should be protected from accumulation of particulate contaminants, which may deposit on the catalyst or otherwise restrict gas flow to and/or through the catalyst. One particular area of concern is the gaps or spaces between individual catalyst blocks. If particulate accumulates in a manner that fills these gaps and bridges the spaces between catalyst blocks, flue gas flow to the surface area of the catalyst is limited, thereby degrading the SCR's relative effectiveness for NOx removal.
This problem has, to date, been addressed by installing thin strips of metal called ash shields (also known as “wall seals”, “catalyst block seals”, or “layer caps”) around catalyst blocks. The size, location and other design parameters of the strips are chosen so that ash remains entrained in the flue gas and does not settle on the one or more catalyst blocks. In general, ash shields are installed by tack welding metal strips to one or more appropriate locations of the catalyst block support frame. Alternatively, or in addition to, ash shields can be tack welded to the SCR casing, both activities being performed after the catalyst blocks are in place. Such ash shields are inconvenient because they are welded into place, prevent screen removal, and need to be removed prior to cleaning and/or replacing the one or more catalyst blocks as screen removal is generally necessary for access the catalyst blocks.
An additional prior practice has been to overlap such ash shields with the catalyst block support frame. However, this is also not desirable because the catalyst blocks need to be cleaned and/or replaced periodically. Catalyst cleaning can be accomplished only if a screen is removed from the top of the catalyst. The screen generally cannot be removed until the ash shield is destructively removed.
In addition to the foregoing, prior art ash shield designs are also subject to installation and operational damage. Welding such ash shields with the catalyst in place also exposes the sensitive catalyst surface to the risk of weld splatter, which can cause catalyst damage and/or degradation.
Given the above, a need exists in the art for devices and methods of diverting particulate contaminants in an SCR without the use of welded ash shields.